Beam pointing and gain correction of large spherical antennas



: July 1; 1969 E. T. PHILLIPS 3,453,633 BEAM POII JTING AND GAIN CORRECTION OF LARGE SPHERICAL ANTENiIAS Filed May 3. 1966 I Sheet 3 of 2 RAY 3 4 IRAY 2 RAY I -x AXIS CENTER 0F 1 F2 PARAXIAL RADIUS F FOCUS Fig.1

ORRECTION DEFLECTION PUT R SIGNALS COM E DATA INVENTOR. EDWARD T. PHILLIPS July'l, 1969 E. T. PHILLIPS 3,453,633

' BEAM POINTING AND GAIN CORRECTION OF LARGE SPHERICAL ANTENNAS Filed May a. 1966 Sheet 3 Gr 2 SIGNAL FEE ELEMENT DEFLECTION DATA M FEED COMPUTER ELEMENT 3s /|4 Fi 3 g 4 FEED ELEMENT DEFLECTION DATA COM Pu ER SIGNAL FEED FEED FEED ELEMENT ELEMENT v ELEMENT 2 3 Fig. 5

3:38 LASER ham United States Patent 3 453 633 BEAM POINTING AND GAIN CORRECTION OF LARGE SPHERICAL ANTENNAS Edward T. Phillips, San Diego, Calif., assignor to The Ryan Aeronautical Co., San Diego, Calif. Filed May 3, 1966, Ser. No. 547,317 Int. Cl. H01q 19/14 U.S. Cl. 343-779 6 Claims The present invention relates to antennas and specifically to beam pointing and gain correction of large spherical antennas.

Large dish type antennas are presently used for radar, space vehicle tracking, satellite communications, radio, astronomy and other uses, requiring accurate control of beam pointing and maximum gain efliciency. Some steerable: antennas are as large as 250 feet in diameter and fixed antennas on the order of 1000 feet in diameter have been built, the latter having beam direction changed by physical or electrical adjustment of the feed. Such large structures are subject to distortion from temperature changes, wind, structural sag or bending, earth movements and other causes. All these distortions change the beam alignment and focal characteristics of an antenna and cause a decrease in efliciency, inaccurate beam pointing, or both.

From a structural aspect, a parabolic antenna of such a large size is very difficult and expensive to build with suflicient accuracy. A spherical reflector ismore desirable, since the curvature is constant and all portions can be made with the same tooling, and surface checking is simplified. Due to spherical aberration, however, the antenna feed must be distributed along a portion of the spherical axis of the reflector in order to utilize the entire reflector surface. While gain changes and beam deflection of this arrangement may not be as pronounced as with a parabolic reflector having a single focal point, any deviations from the optimum are undesirable.

The primary object of this invention, therefore, is to provide means for detecting distortion in a large spherical reflector type antenna and compensating for the distortions by adjusting the antenna feed to maintain beam alignment and gain.

Another object of this invention is to provide distortion compensating means which is operable continuously and is responsive to all distortions in the antenna structure which would affect the accuracy and efficiency.

Another object of this invention is to provide distortion compensating means which is coupled directly to the adaptive line source type feed developed for spherical reflectors.

A further object of this invention is to provide distortion compensating means which is extremely simple and can be made without moving parts, and is mounted in such a manner as to utilize the antenna boresight axis as the basic alignment reference.

The system and its application to a typical antenna are illustrated in the drawings, in which:

FIGURE 1 is a diagram showing the spherical aberration and reference geometry of a spherical reflector;

FIGURE 2 is a diagram of the distortion detecting and compensating system incorporated in an antenna;

Patented July 1, 1969 FIGURE 3 is a view of the antenna face indicating various zones referred to;

FIGURE 4 is a block diagram of a series type phase adjusting means for the feed;

FIGURE 5 is a block diagram of a parallel type phase adjusting means; and

FIGURE 6 is a diagram of a portion of the direction detecting system.

In FIGURE 1 it is seen that in a spherical reflector 10 of radius R, the paraxial focus, or theoretical focus of rays parallel to the axis, is at a distance of one half of the radius from the reflector. Due to spherical aberration, however, all reflected rays do not intersect the paraxial focus, as indicated by three rays at different radial spacings from the axis. Ray 1, near the axis, intersects the axis at focus F1 very close to the paraxial focus on the side toward the reflector. Ray 2, at a register radial spacing from the axis, has a focus at P2 closer to the reflector. The focus P3 of ray 3, spaced further from the axis, is even closer to the reflector. Thus the actual focus of energy striking the reflector parallel to its axis is distributed along a portion of the axis.

To utilize the full reflective surface at maximum efficiency when transmitting or receiving with a spherical antenna, the feed must extend along the portion of the axis encompassing the focal zone of the reflector. This type of feed is known as a line source feed 12 and has multiple, axially spaced feed elements 14, as in FIGURE 2, to cover different zones of the antenna. For purposes of the present description the reflector is considered to be divided, as in FIGURE 3, three concentric zones 1, 2 and 3, each served by a corresponding feed element. The reflector is further divided into four sectors A, B, C and D, of degrees each, the feed 12 having four sides each with three feed elements to cover the reflector surface adequately. It will be evident that any number of sides and elements may be incorporated into the feed, according to the size and performance requirements of the antenna. The feed elements are phased in a particular relationship to each other to realize optimum gain from the antenna. Changes in phasing will change the beam pointing direction of the antenna from its alignment with the axis. This phased or adaptive line source feed is well known as such and reference may be made to a paper by Spencer, R. C., Sletten, C. J., and Walsh, J. E., entitled Correction of Spherical Aberration by a Phased Line Source, Proc. National Electronics Conference, vol. 5, 1959. Another pertinent reference is a paper by Love, A. W., entitled Spherical Reflecting Antennas with Corrected Line Sources, I.R.E. Transactions, vol. AP-lO, No. 5, September 1962.

With reference to FIGURE 1, the three sample rays 1, 2 and 3 are representative of the three hypothetical zones of the antenna. The normal of each incident ray and its reflected ray, from the center of radius, subtends an angle 6 with the axis. From geometric analysis it can be determined that the path length difference 6 between a ray along the axis and ray 1 is:

6 =R[(cos 0 1)-+% (sec 0 -1)] The path length difference 6 between ray 1 and ray 2 is: 5 =R[ (cos 02COS 0 )+Vz-(sec 0 -sec 0 Similarly, the path length difference 6 between my 2 and ray 3 is:

6 =R[(cos -cos 0 /2 (sec (i -sec 0 The feed 12 is tuned to the phase and amplitude distribution along the axis of the reflections representative of the different zones. Any distortion in the antenna will result in changes in this distribution.

In the antenna illustrated diagrammatically in FIGURE 2, an alignment head 16 is mounted substantially at the center of radius of reflector on supporting structure 18. Feed 12 is mounted on the reflector axis on supports 20 from the basic supporting structure 18. Various types of struts, beams, spiders, or other structure can be used for supports. Distortion can occur from changes in spherical curvature of the reflector, axial displacement of the feed relative to the reflector, radial displacement of the feed, or combinations thereof. These can all be caused by temperature variations affecting the structure, or by bending due to wind or atmospheric disturbances. In a steerable antenna the structure can sag or bend under its own weight as the antenna changes position.

The phase errors in the feed caused by distortions can be summarized as follows:

Type of distortion: Phase error Axial displacement of feed, AZ (-1r/A)r AZ. Radial displacement of feed, AS (-21r/)\)(r+r /6+ AS. Error in reflector radius, AR (1r/32h) (r +r /8+ AR.

where r=sin 0/ sin 0 With regard to the error in reflector radius, it has been found that if the errors in the reflected wavefront are not to exceed 16, (an accepted tolerance) then the maximum aperture of a spherical reflector, whose aperture diameter is equal to the radius of curvature, using a simple uncorrected feed at the paraxial focus, is limited to 32 wavelengths of the energy being handled.

In the spherical configuration the ideal point of reference for distortion detection is the center of radius, since the distance from the reference point to the reflector surface is constant and all points on the reflector surfaces are effectively normal to the radius vector. The alignment head 16 thus houses a distortion detecting unit, which is illustrated as optical in nature, although microwave or other energy could be used if necessary. Each sector of each zone of the reflector 10 is provided with a small mirror 22 at a suitable position, as in FIGURE 3. A source of light, indicated in FIGURE 6 as a laser 24, is mounted in the alignment head 16 and has a beam splitter 26 to direct light through individual slits 28 to the respective mirrors 22. The slit images reflected from the mirrors are intercepted by photoelectric sensors 30, one for each mirror 22. Each sensor 30 is capable of detecting deviation of the reflected light beam caused by distortion and providing an electrical error signal representing the deviation. Optical motion detectors of this type are well known and suitable sensors are available. Multiple light sources could be used, but a single low power, continuous or pulsed laser, with suitable beam splitting, is adequate and constant.

The narrow band, high spectral density characteristics of the laser will make the reflected slit images readily identifiable over sunlight or other interfering illumination. In theory the mirrors 22 should ideally be of the same radius as the reflector and can be mounted on or be suitable treated portions of the reflector. With the very narrow beam of the laser and a large radius reflector, however, plane mirrors may be satisfactory.

The error signals from sensors 30 are fed to a computer 32 which provides signals for correcting the phase of the feed elements to compensate for detected distortion. Phase corrections are linear functions of relative feed displacement and reflector radius errors. Thus the computer could be a simple linear mixing matrix with proportional amplifiers to drive the phase correctors. Alternatively, a digital computer could be used, a typical example being the Bunker-Ramo BR-133.

One arrangement is illustrated in FIGURE 4, with computer 32 supplying correction signals to phase shifters 34 and 36 in a series connection with the feed elements. A parallel connected configuration, which might be more desirable for noise characteristics, is shown in FIGURE 5. The phase correctors could utilize varactor diodes, ferrite elements, or other devices with suitable noise characteristics and low loss.

In order to base the distortion on a fixed axis of reference, the alignment head 16 is coaxial with the feed 12 on the spherical axis of reflector 10. To detect any deviation of the feed relative to the alignment head, an additional mirror 22 is mounted on the feed to reflect a. slit image back to a sensor in the optical system.

The reference axis is established in the initial boresighting of the antenna, which is usually accomplished by mounting an optical telescope 38 in axial alignment on the spherical axis of the antenna. The telescope is then pointed at a known substantially point source of combined optical and radio frequency radiation, such as a stellar object. The antenna can then be aligned to the source viewed in the optical telescope, so providing a common axis of reference based on the spherical axis of the reflector, the boresighting technique being well known.

A spherical reflector is simple to construct since all the portions have the same curvature. Tolerances are not especially critical because small deviations from a perfect configuration are compensated for electrically through the system described.

The system provides real-time correction of beam pointing and gain stability, without the need for analysis of distortions or elaborate programming of operations. Structure need not be as rugged as some existing antennas, and no special dome or cover is necessary to protect the antenna from wind and climate changes.

It is understood that minor variation from the form of the invention disclosed herein may be made without departure from the spirit and scope of the invention, and that the specification and drawings are to be considered as merely illustrative rather than limiting.

I claim:

1. In a large antenna having a substantially spherical reflector, with a line source feed mounted on the reflector axis and having a plurality of feed elements directed to selected zones of the reflector, a distortion componensating system, comprising:

a distortion detecting unit mounted substantially at the center of radius of the reflector, said unit having means to observe selected portions of the reflector, detect deviations of the reflector surface relative to the unit and provide error signals corresponding to the deviations;

means to analyze the error signals and provide correction signals representing deviations of the reflector relative to a predetermined reference axis;

and phase adjusting means coupled between said feed elements and actuated by the correction signals to alter the phase relationship of the feed elements to compensate for the distortion.

2. The system according to claim 1 and including means in said distortion detecting unit to detect motion of said feed relative to the unit and provide error signals corresponding to the motion.

3. The system according to claim 1, wherein said means to observe comprises;

a source of wave energy;

means to direct beams of energy from said source to portions of said reflector;

mirror means on said reflector to reflect individual beams back to said source;

5 6 and means at said source to intercept the individual re- 6. The system according to claim 3, wherein said flected beams and sense the deviation thereof from source of wave energy is a laser. a predetermined alignment. 4. The system according to claim 3, wherein each zone References Cited {taf saidt iefietctor regateg toan individual feed element con- 5 UNITED STATES PATENTS ams a eas one 0 sa1 mlrrors.

5. The system according to claim 3 and including means 21??? i ig 3 to direct a beam from said source generally axially to said 9 5 AIS erg 3 8 feed, a mirror on said feed to reflect the beam back to said source, and means to receive the reflected beam and ELI LIEBERMAN Exammer' sense the radial displacement of said feed relative to the 10 f f US. Cl. X.R. ans 0 re erence. 343- 854, 912 

1. IN A LARGE ANTENNA HAVING A SUBSTANTIALLY SPHERICAL REFLECTOR, WITH A LINE SOURCE FEED MOUNTED ON THE REFLECTOR AXIS AND HAVING A PLURALITY OF FEED ELEMENTS DIRECTED TO SELECTED ZONES OF THE REFLECTOR, A DISTORTION COMPONENSATING SYSTEM, COMPRISING: A DISTORTION DETECTING UNIT MOUNTED SUBSTANTIALLY AT THE CENTER OF RADIUS OF THE REFLECTOR, SAID UNIT HAVING MEANS TO OBSERVE SELECTED PORTIONS OF THE REFLECTOR, DETECT DEVIATIONS OF THE REFLECTOR SURFACE RELATIVE TO THE UNIT AND PROVIDE ERROR SIGNALS CORRESPONDING TO THE DEVIATIONS; MEANS TO ANALYZE THE ERROR SIGNALS AND PROVIDE CORRECTION SIGNALS REPRESENTING DEVIATIONS OF THE REFLECTOR RELATIVE TO A PREDETERMINED REFERENCE AXIS; 